1. Field of the Invention
The present invention relates to an ultrasonic flow sensor for detecting a flow of a fluid medium in a flow tube.
2. Description of Related Art
Ultrasonic flow meters based on at least two ultrasonic transducers which are offset in a flow tube in the flow direction and which mutually send ultrasonic signals via at least one reflective surface are known from the related art. Examples of these types of ultrasonic sensors are described in published German patent application documents DE 10 2004 061 404, DE 10 2006 023 479 A1, DE 40 10 148 A1, or DE 10 2006 041 530 A1. Therefore, reference may be made to these publications for possible designs of the ultrasonic transducers and possible measuring principles. For example, sound propagation is accelerated in a flow direction of a fluid medium and retarded in a direction opposite the flow direction. By measuring the ultrasonic propagation time in both directions, the influence of the sound velocity may be compensated for, and the flow rate may be ascertained independently of this sound velocity, which in the case of a gas as the flowing medium is a function primarily of the temperature and the gas composition (for example, the atmospheric humidity), and to a lesser extent, also of the pressure.
Measuring systems are known from the related art in which a reflector may be integrated into a tube wall, or may also be attached to a sensor module, so that, together with the ultrasonic transducers and an electronics system, a type of plug-in sensor results. In this regard, reference may be made to published German patent application document DE 10 2004 061 404 A1, for example. Likewise, multiple nonparallel reflective surfaces may be used in order to detect a larger portion of the flow due to the resulting ultrasonic path. In this regard, reference may be made to published German patent application document DE 43 36 370 C1, for example. Furthermore, a tubular screening of an ultrasonic path with respect to a flowing medium is proposed in published German patent application document DE 10 2006 041 530 A1. The tubular screening is situated in such a way that a tube is oriented in the area of one of the two ultrasonic transducers in such a way that a portion of the transmission path of the ultrasonic wave is guided in this tube. The tube is closed off by one of the ultrasonic transducers, and therefore the ultrasonic propagation distance inside the tube is not part of a measuring segment through which flow passes.
Published German patent application document DE 10 2006 023 479 A1 proposes a curved reflective surface in which the sound is focused toward the respective other ultrasonic transducer, thus compensating for the drift of this ultrasonic wave. This is to be carried out in such a way that drifted beam components reach a reflector section having a different inclination, and as a result of this changed inclination are still deflected to the same or a similar reception point. The reflective surface described in published German patent application document DE 10 2006 023 479 A1 is attached in a flow tube or to a sensor module which also accommodates the ultrasonic transducers. A similar system is also described in published German patent application document DE 10 2004 061 404 A1. In addition, reflective surfaces which sometimes have convex curvatures are known.
However, the ultrasonic sensors known from the related art have a plurality of technical challenges. These result in particular from the fact that different components of the ultrasonic signals are transmitted differently for various flow rates of the fluid medium. Thus, the method described in published German patent application document DE 10 2006 023 479 A1, for example, is based on focusing on and simultaneous compensation for drift of the ultrasonic waves. In practice, however, this compensation is difficult, since different degrees of turbulence and velocity profiles generally form, depending on the flow rate in the flow tube, so that genuine compensation for the drift is possible only in part. In addition, even with full compensation, the reception signal contains ultrasonic components based on different emission and detection angles, depending on the flow rate, which for customary ultrasonic transducers always have slightly different transmission functions. These differences may result in signal distortions, which in particular at higher flow rates significantly hinder unambiguous propagation time measurement. Heretofore, it has been possible to remedy this only by making predictions for the future, at least in principle, based on multiple flow measurements from the past. On the one hand, these predictions are very complicated, and on the other hand may fail when there are interferences or rapid changes in the flow rate.